1. Field of the Invention
The present invention relates to a method for producing a steel strip having at least one surface thereof which exhibits an excellent phosphate-coating property and a satisfactory appearance.
2. Description of the Prior Art
It is known that steel strips electroplated with a zinc-containing metallic material are useful as plate materials for forming car bodies and effectively prolonge the car body life. Surfaces electroplated with a zinc-containing metallic material, however, are not ideal for coating with a lacquer.
A lacquer layer coated over such a surface exhibits poor durability, especially, poor blister resistance. Therefore, the lacquer layer degrades in a short time under severe ambiant conditions.
In order to prevent the degradation of the lacquer layer, car bodies are made from steel strips having only one surface electroplated with a zinc-containing metallic material. This surface forms the inside surface of the car bodies. The other surface is not plated and forms the outside surface of the car bodies. The nonplated surface of the steel strip is effective for preventing the degradation of the lacquer layer, while the plated surface exhibits enhanced resistance to rust.
A recent trend has been for increased thickness of the plated metal layer to enhance the resistance of the steel strip to rust. In the electroplating process, it is known that the amount of electricity necessary for forming a plated metal layer increases with an increase in the thickness of the plated metal layer.
When a steel strip is plated in a continuous plating apparatus provided with a rectifier having a fixed electric capacity, the smaller the moving speed of the steel strip in the continuous plating apparatus, in other words, the longer the contact time of the steel strip with a plating liquid, the larger the thickness of the resultant plated metal layer. That is, in order to provide a large thickness of the plated metal layer, it is necessary to decrease the moving speed of the steel strip in the continuous plating apparatus. This, however, causes decreased productivity of the plated steel strip.
Usually, the electrolyte solution for the electroplating contains an aqueous solution of the sulfuric acid, which is effective for electrically stabilizing the electrolyte. If the continuous electroplating process is applied to only one surface of the steel strip at a reduced speed, the other surface of the steel strip is contaminated with various oxides derived from the electrolyte and is discolored brown, dark brown, or black. This discoloring phenomenon results not only in an undesirable appearance, but also a decreased phosphate-coating property of the other (nonplated) surface of the steel strip.
When a conversion-coating process is applied to the electroplated surface of the steel strip, the other surface of the steel strip is contaminated with a portion of the conversion-coating material. This phenomenon also results not only in an undesirable appearance, but also in a poor phosphate-coating property of the other (nonplated) surface of the steel strip.
As an approach to eliminate the above-mentioned disadvantages, Japanese Examined Patent Publication (Kokoku) No. 55-46470 discloses a method for protecting the nonplated surface of a steel strip in an electroplating process. In this method, the nonplated surface serves alternately as an anode and as a cathode. This method is effective for preventing the discoloration of the nonplated surface of the steel strip.
However, the nonplated surface of the steel strip exhibits a degraded phosphate-coating property, because frequent alternation of the polarity of the nonplated surface of the steel strip results in modification of the surface oxide layer present on the nonplated surface. This surface oxide layer is effective for enhancing the phosphate-coating property of the surface.
Also, in this electoplating process, due to the fact that the electrodes are unevenly consumed and the electrolyte flows at an uneven flow rate in the plating bath, the plated metal layer formed on the surface of the steel strip when it serves as a cathode is not always completely and uniformly removed from the surface when it serves as an anode. This is true even if the amount of current applied to the surface when it serves as a cathode is the same as that when it serves as an anode. That is, the surface of the steel strip not to be plated is sometimes contaminated with residue of the plated metal layer and/or a portion of the surface layer of the steel strip is dissolved. Accordingly, it is very difficult to provide a nonplated surface of the steel strip which is completely free from the plated metal layer and is completely protected from local corrosion thereof.